Systems for detecting and demodulating wavelength division multiplexed (WDM) optical signals have conventionally deployed one of two approaches. One approach spatially separates the frequency components of the optical signal using an optical element such as a diffraction grating so that the frequency components traverse physically different paths to separate detectors. The separate detectors can then decode, demodulate, or otherwise convert the separated frequency components into respective electrical signals. A disadvantage of this approach is that the system must be relatively large to provide space for the separate optical paths and detectors. Another approach employs an array of detectors that are individually much smaller than the cross-section of the WDM signal. A different filter is positioned adjacent to each detector, so that each detector demodulates or converts only a single frequency component that the adjacent filter passes. A disadvantage of this approach is waste of optical power. In particular, the WDM signal must have a cross-section that is large enough to cover the area of multiple detectors, and each detector uses only a small fraction of the light incident on the area of the detector.
Resonant grating waveguide structures have been of interest for light separation and filtering. These structures employ gratings to couple specific wavelengths of incident light into waveguides. A simple configuration for a resonant grating waveguide includes a waveguide layer and a grating layer. The grating layer transmits a part of an incident light beam and diffracts a part of the incident light beam. The diffracted part enters the waveguide layer but through interaction with the grating layer can diffract out of the waveguide layer and interfere with the directly transmitted light. A resonant grating waveguide structure is designed to have a “resonance” such that incident light having a resonant frequency is coupled into the waveguide structure with high efficiently, while incident light at a non-resonant frequency passes through waveguide structure unaltered. The resonant frequency and the bandwidth of the resonance generally depend on the features of the grating and the waveguide layer. However, the bandwidth can be made sufficiently narrow for use in optical filters or separators.
A detector for WDM signals is desired that is compact and efficient in the use of optical power.